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Digestion in stomach

The stomach performs the number of digestive and not digestive functions.

Not digestive functions of the stomach:

1. Participates in regulation of processes of erythrogenesis (Castle’s intrinsic factor)

2. Participates in metabolism

3. Carries out excretory function

4. Carries out endocrine function (there is number of endocrine cells which form peptides of digestive system)

5. Protective (excreted НСl is unfavorable medium for microorganisms)

Digestive functions:

1. Deposition of food

2. Mixing of food (machining)

3. Food is exposed to chemical (enzymatic) processing

4. Portion evacuation into duodenum

5. Adsorption of hydrolysates

Enzymes of acidic gastric juice influence food proteins in relatively narrow zone of food contents which is in direct contact with mucous membrane of the stomach and in small distance from it where gastric juice has diffused and was not neutralized due to buffer properties of food. All mass of food in the stomach is not mixed with juice. As food is fluidified and chemically processed, its layer adjacent to mucous membrane, by motions of the stomach is moved to its antral part and further evacuated into intestine.

а). Secretory activity of stomach. The gastric juice is produced by gastric glands located in mucous membrane. In the region of fornix of the stomach glands contain main glandulocytes (main cells) producing pepsinogens; parietal glandulocytes (coating cells) which synthesize and excrete hydrochloric acid and mucocytes (additional cells), excreting mucoid secret. In pyloric parts of the stomach there is no parietal glandulocytes. Due to difference in the structure of fundus and pyloric glands they produce juice of different structure. The leading role in gastric digestion is played by fundus gastric juice.

In the human stomach it is excreted 2,0-2,5 l of gastric juice a day. It represents pellucid fluid containing hydrochloric acid (0,3-0,5%) and therefore has acidic reaction (рН 1,5-1,8). рН of the stomach chymus is much higher as juice of fundus glands is partially neutralized by the accepted food. The faster the gastric juice is excreted, the lesser it is neutralized and the higher its acidity is.

НСl carries out following functions:

1. Activation of pepsinogen into pepsin

2. Denaturation and swelling of proteins (promotes hydrolysis of proteins)

3. Antibacterial

4. Decalcification of bones

5. Enhances gastric motor activity

6. Stimulates formation of hormones (НСl comes into duodenum, produces hormone of secretin and pancreozymin. These hormones are absorbed into blood, approach pancreas and stimulate it).

7. Evacuation of chymus (performing of obturator pyloric reflex).

b). Structure and properties of gastric juice. In gastric juice there are many inorganic substances: chlorides, sulphates, bicarbonate of natrium, potassium, calcium and magnesium, ammonia. The osmotic pressure of gastric juice is higher than that of blood plasma.

Organic components of gastric juice are represented by the large number of nitrogen-bearing substances (200-500 mg/l): urea, urinary and lactic acids, amino acids, polypeptides. Organic substances are products of secretory activity of stomachal glands and metabolism in mucous membrane of stomach. Enzymes have special value for digestion.

Main glandulocytes ofstomachal glands of person synthesize and excrete pepsinogens of two groups. Pepsinogens of the first group (5 of them) are formed in fornix of stomach, the second bunch (2 of them) - in pyloric part of the stomach and at initial part of the duodenum. Actually, pepsins are considered to be enzymes hydrolyzing proteins with maximal rate at рН 1,5-2,0. Their other fraction hydrolyzes proteins at optimum рН 3,2-3,5 and is called gastricsin. Pepsin and gastricsin impse different kinds of proteins. Pepsins posses the expressed property to coagulate milk. The ability of pepsins in wide range of рН is of great importance in stomachal proteolysis which occurs at different рН depending on the volume and acidity of gastric juice, buffer properties and amount of accepted food. Proteases of gastric juice split proteins up to large polypeptides. Proteins subject to preliminary action of stomachal proteases and formed at it fragments of protein molecules are then more easily split by proteases of juice of pancreas and small intestine.

The gastric juice of an adult person has small lipolytic activity. This lipolytic activity has important value for a child in period of its breast feeding (splitting of emulsificated fat of milk).

The important component of gastric juice are mucoids. Mucus containing mucoids, protects the membrane of stomach from mechanical and chemical irritations.

Glands ofpyloric parts of stomach excrete small amount of juice of alkalescent reaction with big content of mucus. The secret of pyloricglands has small proteolytic, lipolytic and amylolytic activity. Alkaline pyloric secret partially neutralizes acidic contents of stomach evacuated from the stomach into the duodenum.

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